Embodiments of the present invention are directed to power supply control circuits and power supply systems. More particularly, embodiments of the invention provide methods and circuits for controlling an output current in a switched mode power supply (SMPS). Merely by way of example, some embodiments of the invention have been applied to an SMPS constant current controller, such that the output current is substantially independent of variations of AC mains and/or of variations in an output voltage. But it would be recognized that the invention has a much broader range of applicability.
Regulated power supplies are indispensable in modern electronics. For example, the power supply in a personal computer often needs to receive power input from various outlets. Desktop and laptop computers often have regulated power supplies on the motherboard to supply power to the CPU, memories, and periphery circuitry. Regulated power supplies are also used in a wide variety of applications, such as home appliances, automobiles, and portable chargers for mobile electronic devices, etc.
In general, a power supply can be regulated using a linear regulator or a switching mode controller. A linear regulator maintains the desired output voltage by dissipating excess power. In contrast, a switching mode controller rapidly switches a power transistor on and off with a variable duty cycle or variable frequency and provides an average output that is the desired output voltage.
Compared with linear regulators, switching mode power supplies have the advantages of smaller size, higher efficiency and larger output power capability. On the other hand, they also have the disadvantages of greater noise, especially Electromagnetic Interference at the power transistor's switching frequency or its harmonics.
Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM) are two control architectures of switching mode power supplies. In recent years, green power supplies are emphasized, which require higher conversion efficiency and lower standby power consumption. In a PWM controlled switching mode power supply, the system can be forced to enter into burst mode in standby conditions to reduce power consumption. In a PFM controlled switching mode power supply, the switching frequency can be reduced in light load conditions. PFM-controlled switching mode power supply exhibits simple control topology and small quiescent current. Therefore, it is suitable for low cost small output power applications such as battery chargers and adapters.
In such a switched mode power supply system, a switch is connected to the primary winding of the transformer. Magnetic energy is stored in the inductance of the primary winding when the switch is turned on, and the energy is transferred to the secondary winding when the switch is turned off. The energy transfer results in a current flowing through the secondary winding and the rectifying diode. When the energy transfer is completed, i.e., the current stops flowing through the diode, a substantially sinusoidal oscillation of decreasing amplitude appears at the secondary winding. The frequency of the sinusoidal oscillation is determined, in part, by the inductance of the primary winding and by the parasitic capacitance in the primary winding as well in the printed circuit board. The effect of these components often are difficulty to determine in advance and can lead to output performance limitations in the switched mode power supply. Some of these limitations are described in more detail below.
Therefore, there is a need for techniques that can provide more precise control of output current-voltage characteristics in a power supply